The measurement of pressures in geological formations is often of great importance to engineering and environmental matters. To the civil engineer, pore pressures in soils are important in the design of foundations, slopes and retaining walls. To the hydro-geologist, pressures in aquifers and aquicludes are a key to determining groundwater resources and movement. To the petroleum engineer, understanding the pressure of the fluids is critical in determining the resources and reserves of petroleum fluids.
The civil engineering industry often refers to pressure monitoring systems as piezometers. Piezometers take a variety of forms. The most traditional piezometer involves the placement of an open tube standpipe into a borehole with a sand or gravel pack around a slotted tip. A bentonite seal is placed above the gravel pack and the remainder of the hole is cemented. Variations on this theme exist with some standpipes being fitted with a filter tip, where the filter tip is driven into a clay.
The fluid level is generally measured in standpipe piezometers by measuring the water level therein either manually by some form of dipping system, or by the measurement of pressure above a certain point in the standpipe. This has previously been accomplished by measuring the required pressure to force a bubble out of a tube in the standpipe, but is more commonly undertaken by the use of pressure transducers.
The disadvantage of the standpipe system is that the standpipe has a significant volume. To produce a change in the volume of the fluid in the standpipe, fluid must either come out of the formation to fill the standpipe, or pass from the standpipe into the formation. This requires the formation to have an adequate permeability and storage characteristic to operate with the standpipe. This pressure measuring technique also requires a very good connection between the standpipe and the formation. In all cases, the standpipe adversely functions to dampen the true pressures of the formation.
To overcome the volumetric problems with the use of standpipes, low volume pressure transducers were fixed in a filter zone in a borehole or structure. Because of the inherent instability of early electronic devices, pneumatic piezometers were developed. In the use of pneumatic piezometers, two tubes were fitted to the transducer—one to permit the passage of compressed air to the device, and the other to permit the return of the compressed air after it passed through a pneumatic valve. The pressure of the fluid in the formation was detected by the pressure required to pneumatically open the valve, as detected by the airflow up the return tube. This type of transducer was particularly well suited to the monitoring of earth dams as the tubing and transducers could be easily incorporated into the earth structure.
The next major development was to use electrical transducers, particularly of a vibrating wire type. This type of transducer exhibited better long term drift characteristics as compared to the bridge type transducers of the same era. The vibrating wire transducers had very low volumetric requirements to operate an internal diaphragm, and as such were easily incorporated into filter zones within boreholes. The availability of vibrating wire transducers made it possible to install multiple transducers into a single borehole, although this was generally accomplished by the use of multiple levels of gravel packing and cementing.
The next major development was the realisation that in many cases a pressure transducer could be cemented directly into a borehole. To make this possible, the pressure sensing diaphragm of the transducer must be isolated from the direct contact with the cement, and the cement required adequate permeability to permit a fluid connection between the geological formation and the transducer. With this installation method, there is always an uncertainty as to what is connected to what, i.e. is the formation fluid at the same elevation as the transducer in the borehole, or is the fluid in the formation at some other level in the borehole? It has been generally assumed that the pressure measured by the transducer is that of the formation fluid located directly adjacent to where the transducer has been installed. This may not, however, be universally correct as, if the formation adjacent to the transducer is extremely impermeable, and the formation further up the hole is not, then depending on the relative permeabilities of the formations and the cement grout, the pressure measured may not be that produced by the formation located directly adjacent to the pressure transducer. This becomes particularly problematic if shrinkage of the cement grout occurs, which leads to longitudinal leakage paths within the cured grout. When this occurs, the pressure transducer can be influenced by formation pressures that exist above and below the pressure transducer. In this event, the pressure transducer measures the composite of all of the formation pressures to which it is exposed.
Because most exploitable aquifers have high permeability and storage characteristics, the groundwater industry has generally managed to utilise traditional standpipes or the use of monitoring wells. In low permeability formations, investigations have been undertaken to consider low volume fluid pressure measuring techniques.
The petroleum industry is a field where the measurement of geological formation pressures was traditionally accomplished by pressure measurements in test wells or production wells. This situation has since changed dramatically with the introduction of several formation testing tools. Permanent monitoring of formation pressures has also grown with the use of pressure transducers which are fixed in the casing, or to the tubing, having been run into a well and cement grouted into place.
Lastly, it has been proposed that one or more pressure sensing lines could be grouted in the borehole formed in a coal seam to measure the fluid pressures therein. This technique is disclosed in a technical paper published in SPE Reservoir Engineering (February 1987) and entitled ‘Reservoir Engineering in Coal Seams: Part 2—Observations of Gas Movement in Coal Seams’ by Ian Gray. According to this technique, the pressure sensing line(s) is strapped to a PVC conduit and the assembly is lowered into the borehole. The borehole is grouted around the assembly, and the line is filled with water to prevent the grout from flowing up the pressure sensing line. The PVC pipe can accommodate the flow of grout therein. After the grout has set, the pressure sensing line is pressurised to fracture the grout and create an opening to the coal seam. The pressure sensing line can be connected to a pressure gauge or chart recorder located at the surface.
From the foregoing, it can be seen that a need exists for a fluid measuring technique that more accurately measures the fluid pressure in the part of the formation that is at the same depth, elevation or vicinity of the pressure sensor. A further need exists for isolating the pressure sensor in a borehole so that it is only exposed to the fluid pressure in the formation adjacent to the pressure sensor and not to the formation pressure at another position in the hole. A further need exists for a method to isolate the pressure sensor in the borehole using a cement grout between the pressure sensor and the borehole, and then opening a communication path in the cement grout between the pressure sensor and the wall of the borehole where the formation fluid pressure is to be measured. Yet another need exists to undertake the installation of one or more sensors in a single cementing operation.